Enhancement of blastogenesis in cholesterol-enriched lymphocytes.

Studies were carried out to test the ability of the lipid environment to influence the cholesterol content of lymphocyte membranes and to assess the functional significance of variations in lymphocyte membrane cholesterol. Mouse splenic lymphocytes were incubated with cholesterol-lecithin liposomes having a mole ratio of cholesterol to phospholipid (C/P) ranging from 0 to 2.2. After 16 to 20 hr of incubation with cholesterol-rich liposomes (C/P >1.0), the C/P of the membrane fraction of lymphocytes was increased 25%. This was associated with a decrease in membrane fluidity, as assessed by the fluorescence polarization of the hydrophobic probe, 1,6-diphenyl-1,3,5-hexatriene (DPH). Cholesterol-enriched lymphocytes displayed a marked increase in blastogenesis in response to concanavalin A or phytohemagglutinin-M, as evidenced by 3H-thymidine or 3H-leucine incorporation as well as by morphologic evaluation. There was no change in the optimal concentration of lectin for transformation, but the time to maximal thymidine incorporation after concanavalin A was 48 hr in cholesterol-enriched lymphocytes, compared with 72 hr in control cultures. Liposomes exerted a constant effect over a wide concentration range, indicating that it was the C/P ratio of liposomes rather than the cholesterol concentration of the medium that influenced lymphocyte membranes. The maximum effect of cholesterol-rich liposomes was observed after 16 to 20 hr of incubation before lectin addition, with lesser effects induced by liposomes added with lectin, and no effect when liposomes were added 30 hr after the addition of lectin. 7-Ketocholesterol, which inhibits cholesterol synthesis, blocked blastogenesis in response to concanavalin A. However, this effect was reversed by liposomes containing cholesterol, and cholesterol-rich liposomes enhanced lymphocyte blastogenesis even in the presence of 7-ke-tocholesterol. These studies demonstrate that lymphocyte membrane C/P and fluidity are responsive to the C/P of the plasma environment. Cholesterol enrichment of lymphocyte membranes leads to an augmentation of the response of lymphocytes to lectin-induced blastogenesis.